A. Introduction to the Fischer Tropsch Process
The synthetic production of hydrocarbons by the catalytic reaction of carbon monoxide and hydrogen is well known and is generally referred to as the Fischer-Tropsch reaction. The Fischer-Tropsch process was developed in early part of the 20.sup.th century in Germany. It has been practiced commercially in Germany during World War II and later in South Africa.
The Fischer-Tropsch reaction for converting synthesis gas (primarily CO and H.sub.2) has been characterized by the following general reaction: ##STR1##
The hydrocarbon products derived from the Fischer-Tropsch reaction range from methane to high molecular weight paraffinic waxes containing more than 100 carbon atoms.
Numerous catalysts have been used in carrying out the reaction, and both saturated and unsaturated hydrocarbons can be produced. The synthesis reaction is very exothermic and temperature sensitive whereby temperature control is required to maintain a desired hydrocarbon product selectivity.
B. Introduction to Synthesis Gas Production
Synthesis gas may be made from natural gas, gasified coal, and other sources. Three basic methods have been employed for producing the synthesis gas ("syngas"), which is substantially carbon monoxide and molecular hydrogen, utilized as feedstock in the Fischer-Tropsch reaction. The two traditional methods are steam reforming, wherein one or more light hydrocarbons such as methane are reacted with steam over a catalyst to form carbon monoxide and hydrogen, and partial oxidation, wherein one or more light hydrocarbons are combusted sub-stoichiometrically to produce synthesis gas. The steam reforming reaction is endothermic, and a catalyst containing nickel is often utilized. Partial oxidation is the catalytic or non-catalytic, sub-stoichiometric combustion of light hydrocarbons such as methane to produce the synthesis gas. The partial oxidation reaction is typically carried out using high purity oxygen. High purity oxygen, however, can be quite expensive.
In some situations these synthesis gas production methods may be combined to form the third method. A combination of partial oxidation and steam reforming, known as autothermal reforming, wherein air (or O.sub.2) is used as a source of oxygen for the partial oxidation reaction has also been used for producing synthesis gas heretofore. Autothermal reforming is a combination of partial oxidation and steam reforming where the exothermic heat of the partial oxidation supplies the necessary heat for the endothermic steam reforming reaction. The autothermal reforming process can be carried out in a relatively inexpensive refractory lined carbon steel vessel whereby low cost is typically involved.
The autothermal process results in lower hydrogen to carbon monoxide ratio in the synthesis gas than does steam reforming alone. That is, the steam reforming reaction with methane results in a ratio of about 3:1 or higher while the partial oxidation of methane results in a ratio of less than about 2:1. A good ratio for the hydrocarbon synthesis reaction carried out at low or medium pressure (i.e., in the range of about atmospheric to 500 psig) over a cobalt catalyst is about 2:1. When the feed to the autothermal reforming process is a mixture of light shorter-chain hydrocarbons, such as a natural gas stream, some form of additional control is required to maintain the ratio of hydrogen to carbon monoxide in the synthesis gas at the optimum ratio (for cobalt based FT catalysts) of about 2:1. For this reason steam and/or CO.sub.2 may be added to the synthesis gas reactor to adjust the H.sub.2 /CO ratio to the desired value with the goal of optimizing process economics.
C. Introduction to F-T Reactors and Techniques
Numerous types of reactor systems have been used for carrying out the Fischer-Tropsch reaction. The developed Fischer-Tropsch reactor systems have included conventional fixed-bed, three-phase slurry bubble column designs, fluidized and/or moving bed, and ebullating beds to name a few. Due to the complicated interplay between heat and mass transfer and the relatively high cost of Fischer-Tropsch catalysts, no single reactor design has dominated the recent commercial development efforts.
Fixed-bed reactors have individual catalyst particles (typically less than 15 mm in the characteristic diameter) packed into tubes in cylindrical vessels. The individual particles of various shapes such as trilobes, spheres or cylinders, typically contain voidages on the order of about 0.3 to 0.5 depending upon the specific particle shape. These reactors offer simplicity and conversion kinetics that are easy to scale up to commercial sized units. Due to the high heat release and relatively low mass velocity associated with commercial operations, however, the reactor tube sizes are kept relatively small (typically less than two or three inches) when operating with a gas continuous system.
Fixed-bed Fischer-Tropsch reaction systems are primarily constrained by pressure drop and heat transport limitations. High productivity and good methane selectivity generally can be achieved with small particle sizes, typically on the order of less than 200 microns. In this context, "selectivity" refers to the following ratio: (moles of referenced product formed)/(mole of CO converted). The pressure drop, however, limits the practical application to much larger particle sizes for use in fixed-bed reactor systems. Shaped extrudates (trilobes, quadralobes, etc.) in the range of about 1/64 to 1/8 inch in diameter are frequently used. Smaller size extrudates are infrequently used because they are difficult to manufacture in commercial quantities and create high pressure drops across the bed.
The heat transfer characteristics of such fixed-bed reactors are generally poor because of the relatively low mass velocity. If one attempts, however, to improve the heat transfer by increasing the velocity, higher CO conversion, which is the commercial goal, can be obtained but there is an excessive pressure drop across the reactor, which limits commercial viability. In order to obtain the CO conversions desired and gas throughputs of commercial interest, the needed conditions result in a high radial temperature profile. Due to the high heat of reaction, Fischer-Tropsch fixed-bed reactor diameters are generally less than three inches to avoid excessive radial temperature profiles. The use of high-activity catalysts in Fischer-Tropsch fixed-bed reactors, which must handle a large exothermic heat of reaction and which have poor effective thermal conductivity in the packed bed, may cause large radial temperature profiles to exist.
Further, the use of catalyst particle sizes greater than 1/64 of an inch to avoid excessive pressure drop through the reactor results in high methane selectivity and low selectivities toward the high molecular weight paraffins, which generally have more economic value. This selectivity is due to a disproportional catalyst pore diffusion limitation on the rate of transport of reactants--CO and H.sub.2 --into the interior of the catalyst pellets. To address the situation, the use of catalysts having the active metal component restricted to a thin layer about the outer edge of the particle has been suggested. These catalysts appear costly to prepare and do not appear to make good use of the available reactor volume. Other fixed-bed reactor system alterations have also been proposed.
U.S. Pat. No. 5,786,393 presents the use of liquid recycles as a means of improving the overall performance in a fixed-bed design. This art has been referred to by some as a "trickle bed" reactor (as part of a subset of fixed-bed reactor systems) in which both reactant gas and an inert liquid are introduced (preferably in an upflow or down flow orientation with respect to the catalyst) simultaneously. The presence of the flowing reactant gas and liquid improves overall reactor performance with respect to CO conversion and product selectivity. In a number of respects, however, the trickle-bed reactor system remains limited.
A limitation to the trickle bed system as well as to any fixed-bed design is the pressure drop associated with operating at high mass velocities. The gas-filled voidage in fixed-beds (typically &lt;0.45) does not permit high mass velocities without excessive pressure drops. What is considered an "excessive pressure drop" will vary with the reactor and situation based primarily on operational cost concerns, e.g., compressor sizing and cost, and catalyst pellet strength concerns because too high a pressure drop can cause particle attrition/crushing. Consequently, the mass throughput undergoing conversion per unit reactor volume is limited due to the heat transfer rates. Increasing the individual catalyst particle size may slightly improve heat transfer by allowing higher mass velocities (for a given pressure drop), but the loss in selectivity toward the high boiling point products and the increase in methane selectivity combined with the increase in catalyst activity generally offset the commercial incentives of higher heat transfer.
In addition to the heat transfer limitations associated with fixed-bed reactors, Fischer-Tropsch catalyst performance is sensitive to mass transfer limitations within the reactor and the individual catalyst volume. It is known that Fischer-Tropsch product selectivity is sensitive to the H.sub.2 /CO feed ratio. Increasing this ratio leads to poor selectivity (i.e. high methane and lower boiling point liquids), but the catalyst productivity, which may be indicated by the expression: (volume CO converted)/(volume of catalyst-hour), increases. In fixed-bed operations that employ large catalyst particles with relatively long diffusion lengths, the H.sub.2 /CO ratio within the catalyst volume can change significantly. Consequently when utilizing larger catalyst particles to mitigate pressure drop and improve the heat transfer (through increasing mass velocity), the performance of the Fischer-Tropsch fixed-bed catalyst systems may degrade due to longer intra-particle diffusion distances resulting in increasing H.sub.2 /CO ratios. This degradation influences performance through lower productivities and lower selectivities towards higher-valued products.
Fischer-Tropsch three-phase bubble column reactors generally offer advantages over the fixed-bed design in terms of heat transfer and diffusion characteristics. Numerous designs that incorporate small catalyst particles suspended by the upflowing gas, which bubbles through a liquid continuous matrix. In this design, reactor diameters are no longer limited by heat transfer characteristics. The motion of the liquid continuous matrix allows sufficient heat transfer to achieve higher commercial productivity (e.g., &gt;200 vol. CO converted/vol. Cat.--hour). The catalyst particles are moving within a liquid continuous phase, resulting in high heat transfer from the individual particles, while the large liquid inventory in the reactor provides a high degree of thermal inertia, which helps prevent rapid temperature increases that can lead to thermal runaway. Further, the small particle size minimizes the negative impact of diffusional resistances within the interior of the catalyst.
The major technical issues associated with three-phase bubble columns include hydrodynamics and solids management. Reactor parameters should be selected to allow sufficient gas/liquid contacting while operating at gas throughputs that achieve the desired residence time and CO conversion levels. In this reactor type the H.sub.2 and CO reactants should transfer from the feed gas (bubbled into the reactor volume) into the liquid phase. Once in the liquid phase, the dissolved reactants contact the catalytic surface to undergo reaction. The transfer of reactants from the liquid phase to the catalyst surface depends upon the turbulence of the liquid continuous phase and the diffusional length to the catalytic surface. Smaller catalyst particles are preferred in slurry reactors to avoid mass transfer limitations that lead to unacceptable product selectivity.
Small particles can be used in these systems because they are readily fluidized by the gas flow. The pressure drop across the reactor is limited to approximately the static head of the bed. Small particles, because of their large surface area also result in improved liquid-solid mass transfer compared to fixed-bed Fischer-Tropsch hydrocarbon synthesis reactors. Ultimately, the particle size is limited by the solids management system. The large ratio of liquid volume to catalyst used in slurry reactor systems provides a large reservoir of dissolved H.sub.2 and CO, which improves operability of the catalyst system.
Liquid-phase back-mixing, however, which is reported to be a strong function of reactor diameter, can result in a much lower kinetic driving force that requires more reactor volume than a fixed-bed reactor operating at the same conversion. The need to have sufficient gas-liquid-solid mixing and liquid-solid separation complicates the equipment requirements and scale-up issues associated with commercial designs.
With respect to the latter, solids management issues, there are number of issues that complicate slurry reactor systems. First, the gas distributor itself can be a major issue. A distributor is desired that distributes in a more or less uniform manner across a potentially very large diameter while preventing "dead" zones in which the catalyst can settle out/down and lay on the reactor bottom. The reactor bottom itself may be the distributor. Second, catalyst/wax separation can be a significant technical hurdle, which limits minimum catalyst particle size and can be very negatively impacted by catalyst particle attrition--especially over long time periods and/or in concert with poorly designed gas distributors.
Commercial designs of fixed-bed and three-phase slurry reactors typically utilize boiling water to remove the heat of reaction. In the fixed-bed design, the individual reactor tubes are located within a jacket containing water. The heat of reaction raises the temperature of the catalyst bed within each tube. This thermal energy is transferred to the tube wall forcing the water to boil within the jacket. In the slurry design, tubes are typically placed within the slurry volume and heat is transferred from the liquid continuous matrix to the tube walls. The production of steam within the tubes provides the needed cooling. The steam in turn is cooled/condensed in another heat exchanger outside of the reactor. All the required heat removal devices can be a considerable capital cost in building a commercial plant as well as the source of numerous technical difficulties.
Fluidized bed type Fischer-Tropsch reactors also give much better heat transfer characteristics than fixed bed reactors and can employ very small catalyst particles. These reactors operate essentially "dry", which means that the production rates of species which are liquid at reactor conditions must be very low, approaching zero. Otherwise, rapid catalyst defluidization can occur. In practice, this requires very high reactor operating temperatures, which typically lead to high selectivities to methane and the production of a number of less desirable chemical species, such as aromatics. Catalyst/gas separation can also be a significant technical and economic hurdle with fluidized bed systems.
A reactor system has been proposed in PCT Application WO 98/38147 that uses a parallel-channel monolithic catalyst support to provide a fixed, dispersed catalyst arrangement. The embodiments discussed and presented include a catalyst with elongated monolithic support (e.g., 10 cm axial length) with active metals incorporated into lengthwise channels. The application contemplates using this catalyst in a Taylor flow regime. "Taylor flow regime" typically signifies a small capillary flow having a large axial dimension compared to the effective radial dimension, e.g., L/D&gt;1000. It is such that entrance effects are not a real factor-it is not regarded as very turbulent. A Taylor flow of gas and liquid in a channel may be defined as periodic cylindrical gas bubbles in the liquid having almost the same diameter as the channel and without entrained gas bubbles between successive cylindrical bubbles.
D. Improved Economics Desired
It has been a quest for many to improve the economics of processes utilizing the Fischer-Tropsch reaction. Improved economics will allow for wide-scale adoption of the process in numerous sites and for numerous applications. Efforts have been made to improve economics, but further improvements are desirable. See for example U.S. Pat. Nos. 4,883,170 and 4,973,453, which are incorporated by reference herein for all purposes.